I'm confused: .223 cal. or 5.56mm?

This is a discussion on I'm confused: .223 cal. or 5.56mm? within the Defensive Rifles & Shotgun Discussion forums, part of the Related Topics category; I have a Colt Match Target HBAR (a post-ban AR-15, essentially: no flash suppressor, no bayonet lug, no collapsible stock).
The rifle's receiver is marked ...

I'm confused: .223 cal. or 5.56mm?

I have a Colt Match Target HBAR (a post-ban AR-15, essentially: no flash suppressor, no bayonet lug, no collapsible stock).

The rifle's receiver is marked "Cal. 223". (That'd be what I'd call "my first clue," I guess.)

I have always used Remington .223 ammunition, which I bought in a case, for shooting at the range (although I have not had the pleasure of doing that in a long time). The ammuntion I have says .223 cal., but I am confused because it seems to me that people say .223 and 5.56mm interchangeably. But I thought I read a thread on some board that said they are not interchangeable.

Can someone set me straight? Are they essentially the same thing, or are they not, and should I not ever use anything called 5.56mm in my rifle, like for instance if I found a good deal on it?

A man fires a rifle for many years, and he goes to war. And afterward he turns the rifle in at the armory, and he believes he's finished with the rifle. But no matter what else he might do with his hands - love a woman, build a house, change his son's diaper - his hands remember the rifle.

Answer asked and made . For the new guys on the thread unless and untill you know better if your rifle is marked .223 shoot that only the 5.56 ammo can be just enough bigger to harm your rifle if the chamber is off .

Make sure you get full value out of today , Do something worthwhile, because what you do today will cost you one day off the rest of your life .
We only begin to understand folks after we stop and think .

The receiver marking has nothing to do with the barrel chamber. If it's a complete Colt factory rifle, the barrel will be marked with the chamber dimensions. It will probably look something like: C MP 5.56mm NATO 1/9 HBAR

223 or 5.56 ammo

I came across this quote:

"Using commercial .223 Remington cartridges in a 5.56mm NATO chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223 Remington chambered gun due to the longer leade.[9] Using 5.56mm NATO mil-spec cartridges (such as the M855) in a .223 Remington chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and the SAAMI recommends against the practice.[10][11] Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56mm NATO, such as many commercial AR-15 variants and the Ruger Mini-14, but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure.223 or 5.56 ammo (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56mm NATO ammunition.["